(1) Field of the Invention
The present invention relates generally to a novel structure of a heat pipe which is applicable to many fields to which conventional heat pipes cannot be applied.
(2) Background of the Art
In the structure of a previously proposed cylinder type heat pipe, a working liquid is sealed within a cylindrical container and is heated and gasified at a heat receiving portion (vaporization portion) to form a vapor stream. Then, the vapor stream is raised toward a heat radiating portion (condensation portion) at a high speed.
At this time, the vapor stream is cooled and liquefied to form a working liquid stream. The working liquid, in turn, circulates toward the heat receiving portion by means of a capillary action of a wick in the container.
In this way, due to a latent heat caused by the vaporization and condensation of the working liquid during such a circulation cycle in a liquid phase and gas phase as described above a heat transfer of the cylinder-type heat pipe is carried out.
In the above-described type of the heat pipe, the working liquid and its vapor flowing in mutually opposite directions are in direct contact with each other.
On the other hand, a Japanese Patent Application First (Non-examined) Publication Sho 60-178921 published on Sept. 12, 1985 exemplifies a structure of a loop-type heat pipe. Almost all parts of a close-looped flow passage of the working liquid in the container are filled with a wick. When the heat receiving portion receives heat, the vapor generated in the wick having an end in the heat receiving portion is sprayed out toward a non-wick portion which has less fluid resistance to form the vapor stream. Then, the vapor stream is moved to the heat radiating portion and liquefied therein. The liquefied stream is then absorbed in the wick by means of the capillary action of the wick. Thus, the liquefied working liquid is recirculated to the heat receiving portion.
The loop-type heat pipe carries out the heat transfer due to the latent heat caused by a change in the phases (liquid phase and gas phase) of the filled working liquid in the circulation cycle described above in the same way as the cylinder-type heat pipe.
The above-described previously proposed structures of both cylinder-type and loop-type heat pipes have, however, the following disadvantages.
(a) Less amount of heat transport due to the presence of low limit of heat transfer.
Mutual interference between the vapor stream and liquid stream occurs due to opposite flow directions of the vapor and working liquid streams in the case of the cylinder-type heat pipe.
When a temperature difference between the heat receiving portion and heat radiating portion is increased, speeds of the vapor stream and working liquid stream are increased respectively. At this time, the working liquid evaporates from an intermediate portion of a wick surface. The working liquid is then blown up and scattered around from the wick surface toward the heat radiating portion. The scattered vapor stream disturbs the recirculated working liquid. Thus the amount of the recirculated working liquid toward the heat receiving portion is reduced. Finally, the working liquid is dried out.
In the case of a wickless-type heat pipe, the above-described phenomenon occurs at an earlier stage and more violently than the wick-type heat pipe. Therefore, the previously proposed cylinder-type heat pipe has a disadvantage of reaching a limit of heat transfer operation by relatively small amounts of heat transportation. As the length of the heat pipe is long or inner diameter of the heat pipe is small, the above-described phenomenon occurs at the earlier stage.
To avoid the above-described phenomenon, a heat insulating portion of the container can be constructed in a double pipe structure. However, the above-described double pipe structure becomes complex and very expensive.
(b) Inevitable presence of a wick limit.
In the case of the wick-type heat pipe, a thermal resistance value is low as a heat input is low and the pipe exhibits a good performance characteristic. However, if the heat input becomes large, boiling and vaporization of the working liquid are generated inside the wick. Therefore, since the recirculated working liquid cannot flow into the heat receiving portion of the wick and consequently becomes dried out. This is called a wick limit. Such a phenomenon is easy to occur as capillaries of the wick become thinner and thickness of the wick becomes thicker.
(c) The occurrence of abnormalities due to a water hammer action.
If a quantity of working liquid is increased in the case of the wickless-type heat pipe, a maximum heat transfer rate can become larger by a multiple number as compared with the wick-type heat pipe. However, if an abrupt heat input or large heat input is applied, the working liquid boils violently. Consequently, the working liquid still in the liquid phase is blown up toward the heat radiating portion and violently collides with the end surface of the heat pipe.
In this case, the heat transportation of the wick-type heat pipe becomes intermittent. In addition, an abnormal sound and an abnormal vibration are generated. In the case of the violent collision, the heat pipe container is often damaged. Such a phenomenon as described above is generated if the quantity of working liquid is too much.
(d) Presence of limits in the length and diameter of the heat pipe.
As an inner diameter of the heat pipe becomes smaller due to mutual actions of a liquid resistance and wick limit in the heat insulating portion, a limit length of the heat pipe becomes shorter. The limit length of the heat pipe having the inner diameter of 20 mm is about 10 meters and that of the heat pipe having the inner diameter of 2 mm is about 400 mm.
(e) Limited mounting orientations of the whole heat pipe during its application.
When the above-described heat pipe is used under a top heat situation, i.e., in a state where a water level of the heat receiving portion is higher than that of the heat radiating portion, even the wick-type heat pipe has a remarkably reduced heat transportation capability.
If the water level difference exceeds about 500 mm, the heat pipe becomes dried out and cannot be used any more. The thermal resistance value becomes doubled even in the horizontal posture. If the heat input becomes increased, the dry out of the working liquid easily occurs. Hence, the heat pipe is commonly used in a bottom heat state (i.e., the water level of the heat receiving portion is lower than that of the heat radiating portion) with a tilting angle of 15 to 20 degrees with respect to the horizontal direction. The wickless-type heat pipe cannot be used in the horizontal direction.
It is noted that the wickless-type heat pipe cannot function any more when the mounting orientation thereof is under the top heat situation.
(f) Difficulty in mountings on heated and cooled objects.
No flexibility is present in the above-described containers and it is almost impossible to use a product of the heat pipe without the product being bent. Hence, it is difficult or impossible to mount on the heated object and on the cooled object. If the container is formed in a corrugated-pipe configuration to provide the flexibility for the heat pipe, the heat pipe does not only become expensive but also fluidity of the working liquid becomes reduced. Consequently, the performance of the heat pipe becomes worsened.
(g) Difficulty arises in the sealing operation of the working liquid in the container.
In a case where a non-condensative gas in the container is generated or mixed, the non-condensative gas during the operation of the heat pipe stays within the heat radiating portion and the performance of the heat pipe can, thus, remarkably be reduced. To prevent such a reduced performance, a finest attention needs to be paid to maintain a high vacuum state of the heat pipe during the sealing-in operation of the working liquid.
(h) No occurrence of mutual interference in the working liquid stream in the case of the loop-type heat pipe disclosed in the above-identified Japanese Patent Application Publication.
Hence, item (a) of the above-described problems can be solved.
On the other hand, since the working liquid vaporizes within the wick, no such sudden boiling as in the case of the wickless-type pipe occurs.
Therefore, item (c) of the above-described problems can be solved.
In addition, the recirculation of working liquid toward the heat receiving portion is carried out only by means of the capillary action of the elongate wick. The long distance of the elongate wick causes the action of weight to be almost offset by means of a viscous resistance force within the filled wick. Hence, the performance difference of the loop-type heat pipe between its horizontal posture and vertical-bottom posture (the heat receiving portion is vertically placed below the heat radiating portion) described in item (e) of the above-described problems can be improved.
However, it is impossible for the loop-type heat pipe disclosed in the above-identified Japanese Patent Application to solve the problems other than items (a), (d) and (e). The loop-type heat pipe, in turn, further worsens the problems.
That is to say, at the liquid recirculation side of the heat insulating portion, the fluid resistance caused by the wick violently increases and item (b) of the problems becomes further worsened. In addition, it is extremely difficult to form the elongate wick in such a small-diameter heat pipe. Furthermore, since the loop-type heat pipe in which the vaporization of the working liquid is carried out within the wick, the problem of item (c) becomes worser and the dry out easily occurs. The problem that the heat pipe cannot almost be used under the top heat situation at the level difference exceeding 500 mm described in item (e) cannot be solved any more.
Item (f) cannot be solved. Since the disclosed loop-type heat pipe may more or less improve the item (g), there is a possibility of staying the non-condensative gas within the wick. In this case, the capillary action will be reduced and the performance of the heat pipe is thereby deteriorated.
As a new problem added to the disclosed loop-type heat pipe, the flow speed of the recirculated working liquid is determined only by means of the transport capability by means of the capillary action of the wick. Therefore, the heat transfer capability in terms of a diameter ratio of the heat pipe may not be improved more remarkably than the cylindrical heat pipe structure.
Japanese Patent Application First (Non-examined) Publications sho 62-252892 published on Nov. 4, 1987 and sho 63-49699 published on Mar. 2, 1988 exemplify the previously proposed structures of the loop-type heat pipes. Although the basic concepts of the loop-type heat pipes may be similar to that in the present invention, the present invention remarkably improves the structure of the loop-type heat pipe.